The present invention relates to the manufacture of optical waveguide fibers.
Optical waveguide fibers have been greatly improved during the last decade. Fibers exhibiting very low losses are generally formed by chemical vapor deposition (CVD) techniques which result in the formation of extremely pure materials.
In accordance with one embodiment of the CVD technique, often referred to as the inside vapor phase oxidation process, the reactant vapor and an oxidizing medium, flow together through a hollow substrate tube where they react to form glass layers. The resultant preform is collapsed and drawn into a fiber.
In another embodiment of the CVD process, the vapor of reactant compounds is introduced into a flame, a laser beam or the like where it is oxidized to form a glass particulate material or soot which is directed toward a mandrel. This so-called flame hydrolysis or outside vapor phase oxidation method of forming coatings of glass soot is described in greater detail in U.S. Pat. Nos. 3,737,292; 3,823,995; 3,884,550; 3,957,474 and 4,135,901. To form a step-index optical waveguide fiber a first soot coating is applied to the mandrel, and thereafter, a second soot coating having a lower refractive index than the first is applied over the outside peripheral surface of the first coating. To form a gradient index fiber, a plurality of layers of glass soot are applied to the mandrel, each layer having a progressively lower index of refraction as taught in U.S. Pat. No. 3,823,995. Gradient index fibers may also be provided with a coating of cladding material. After the plurality of coatings are formed on the mandrel, the mandrel is generally removed, and the resultant tubular porous preform is gradually inserted into a consolidation furnace, the temperature of which is sufficiently high to fuse the particles of glass soot and thereby consolidate the soot preform into a dense glass body in which no particle boundaries exist. In one embodiment of the outside vapor phase oxidation process, which is described in U.S. Pat. Nos. 3,957,474 and 4,486,212, the starting rod forms the core of the resultant fiber. The deposited cladding soot is consolidated on the surface of the core rod. The resultant consolidated preform is drawn into an optical waveguide fiber.
Although CVD techniques for forming optical waveguide preforms result in the formation of optical waveguide fibers having extremely low attenuation, they are relatively expensive. Fiber manufacturing cost can be lowered by increasing preform size and by increasing deposition rate.
The size of preform which can be formed by the inside vapor phase oxidation process is relatively limited. The length of the hollow cylindrical substrate tube is limited to that length which can be supported between two separated chucks while being heated to reaction temperature. The substrate tube diameter is also limited in that process.
The outside vapor phase oxidation technique readily lends itself to cost reduction modifications. Initially, preforms were made larger by increasing the diameter. This was accomplished by traversing the burner longitudinally back-and-forth along the soot preform more times and adding thereto additional layers of increasing radius. Preform length was increased by supporting the preform vertically during deposition to prevent the longer length preform from sagging. Also, a plurality of burners were used to simultaneously deposit soot on a preform. Two burners positioned side-by-side have been moved in unison to simultaneously deposit soot on a preform. When the pair of burners stops traversing as the first burner reaches the end of the preform, the second burner is short of the preform end by the burner-to-burner spacing. The resultant "end effect" necessitates the discarding of the portion of the preform that has been formed by only one of the burners. If the two burners independently traverse the preform and each of them stops traversing the preform at the end thereof, the paths of the burners can cross. Under certain circumstances, the resultant interference can cause solid or gaseous inclusions and composition control problems. If many burners are employed and each is traversed back and forth along only a segment of the entire preform, the soot buildup is not uniform throughout the entire length of the preform since all burners cannot provide precisely the same composition and amount of soot.